CustomFirmwarePlugin: Expose Gripper command

Expose MAV_CMD_DO_GRIPPER Mission item to Mission planning GUI and
update the name shown to the user

This is used for the Package Delivery feature of PX4

src/FirmwarePlugin/PX4/PX4FirmwarePlugin.cc

@@ -283,6 +283,7 @@ QList<MAV_CMD> PX4FirmwarePlugin::supportedMissionCommands(QGCMAVLink::VehicleCl
         MAV_CMD_NAV_DELAY,
         MAV_CMD_CONDITION_YAW,
         MAV_CMD_NAV_LOITER_TO_ALT,
+        MAV_CMD_DO_GRIPPER
     };
 
     QList<MAV_CMD> vtolCommands = {

src/MissionManager/MavCmdInfoCommon.json

@@ -988,8 +988,8 @@
         {
             "id":                   211,
             "rawName":              "MAV_CMD_DO_GRIPPER",
-            "friendlyName":         "Gripper",
-            "description":          "Operate EPM gripper.",
+            "friendlyName":         "Gripper Mechanism",
+            "description":          "Control a gripper mechanism.",
             "specifiesCoordinate":  false,
             "friendlyEdit":         true,
             "category":             "Advanced",